A: Check item meta-stats under Community for a list of weapons; the rarest ones are best.

You may mean, "which is the best weapon I can buy RIGHT NOW" in which case the answer is, whichever gives you the best damage/cost ratio. If you have only a little money, pick the best from the store. If you have more money, pick the weapon with the highest base damage and upgrade it at the blacksmith. If you have a LOT of money, find someone to sell you one of those rares... :)